In recent years, there has been realized a system which has a semiconductor integrated circuit arranged at every location in life and can use an information terminal at any time in any place, that is, a ubiquitous system.
A cellular phone and a wireless LAN (local area network) terminal are an example of the information terminal used in such system. These are of portable type and incorporate a battery. Due to requirement for reducing consumption of the battery, power consumption of the devices is reduced. For instance, a cellular phone which can change transmitted electric power according to communication distance to reduce power consumption is disclosed in Japanese Patent Laid-open No. 2000-224105.
A wireless tag and a contactless IC card on which an IC chip and an antenna are mounted are another example of the information terminal and have been handily used recently. The wireless tag and IC card have specific data and supply the data to a reader and writer. With no power source, they receive an electromagnetic wave emitted from the reader and writer and use as a power source an electric power obtained by rectifying the wave. The obtained electric power is weak, and power consumption of the circuit should be reduced. To make it possible to use a memory operated with low power consumption at low voltage, an example of a contactless IC card which divides a rectified electric power into two for use is disclosed in Japanese Patent Laid-open No. Hei 11 (1999)-73481.
An example of a wireless terminal of a non-portable type which starts to operate only when needed for reducing power consumption to intermittently transmit sensor information is disclosed in Japanese Patent Laid-open No. Hei 11 (1999)-287818.